Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Bylde - Bylde is a SaaS platform you can use to Start, Manage and Grow your groups and communities.
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.